This invention relates to a photomultiplier assembly and particularly to an assembly in which a photomultiplier tube is aligned within a magnetic shield.
All photomultiplier tubes are sensitive, to some extent, to the presence of external magnetic and electrostatic fields. These fields may deflect electrons from their normal path between stages of the photomultipler tube and cause a loss of gain. Tubes designed for scintillation counting are generally very sensitive to magnetic fields because of the relative long path from the photocathode to the first dynode of the photomultiplier tube; consequently, such tubes ordinarily require electrostatic and magnetic shielding. Magnetic fields may reduce the anode current of the photomultiplier tube by as much as 50 percent or more of the "no-field" value.
High-mu material i.e., material of high magnetic permeability, in the form of foil or preformed shields is available commercially for most photomultiplier tube shielding applications. When such a shield is used, it is generally operated at photocathode potential. In scintillation counting applications it is recommended that the photocathode be operated at ground potential so that a potential gradient does not exist across the glass faceplate of the photomultiplier tube. Such a potential gradient would cause scintillations to occur within the glass and increase the dark current of the tube.
One method used to shield the photomultiplier tube is to encapsulate the tube, including its voltage divider network, within a flexible silastic potting material and to attach the encapsulated tube and network with an adhesive to the interior surface of a mu-metal magnetic shield. Such a structure is satisfactory for operating temperatures near room temperature, however, the above described encapsulated structure is unsatisfactory for operation in a high temperature environment. At temperatures of about 150.degree. C. the silastic encapsulating material expands more rapidly than either the glass envelope of the photomultiplier tube or the mu-metal shielding and the resultant force compresses and ruptures the photomultiplier tube.